Native glutamine synthetase preparations containing an intermediate number (2 to 10) of adenylylated subunits are complex mixtures of molecules that differ from one another with respect to the number of adenylylated subunits they contain. By means of affinity chromatography on Affi-blue sepharose columns or on anti-AMP antibody columns, such preparations have been resolved into a series of more or less homogenous fractions, each containing a specified number of adenylylated subunits. In addition, multiple enzyme species containing the same number of adenylylated subunits, but which differ from one another in their affinities for ADP and/or sepharose bound Cibacron dye, have also been obtained. These presumably represent isomeric forms that differ in the distribution of adenylylated and unadenylylated subunits within the dodecameric structure.